It is known that gas-phase acoustic fields induce local pressure differences around the surfaces of liquid droplets (King, L. V. Proc. Roy. Soc., A147, 212-240 (1934)). The forces produced by acoustic standing waves are significantly larger than those produced by travelling waves. Acoustic standing waves can be produced either using an ultrasound transmitter (sonotrode) and a reflector or by using a pair of ultrasound transmitters.
It is known to cause droplets to be levitated in the nodes of standing-wave ultrasound fields. Similar techniques are used for lateral stabilization of droplet position in wind tunnels in which droplets are suspended vertically by balancing gravitational and aerodynamic forces (Lupi, V. D., Hansman, L. J. Journal of Atmospheric and Oceanic Technology, 8, 541-552, (1991)). Under appropriate conditions, droplet breakup can also be induced, and this effect can be used to atomize streams of liquid.
It is also known, for example, to use an ultrasonic standing wave arrangement to produce a paint spray mist for painting a workpiece.
It is desired to provide an improved ion source and method of ionising a sample.